Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy.
However, engine run time in hybrid vehicles (HEVs) may be limited, thus limiting engine manifold vacuum, which is typically used to draw fresh air through the fuel vapor canister to desorb the stored fuel vapors. Rather than continuously routing fuel tank vapors to the fuel vapor canister, the fuel tank may be coupled to a fuel tank isolation valve (FTIV) which may trap fuel vapor within the fuel tank, releasing the fuel vapor to the canister only prior to and during refueling events. In this way, bleed emissions can be reduced, as the resting fuel vapor canister load can be minimized.
However, by sealing the fuel tank, fuel vapor concentrations will increase as the fuel temperature increases. This may be due to increases in ambient temperature (e.g. over a diurnal cycle) and/or due to heat rejected to the fuel tank from the engine and exhaust system during engine operations. As a result, when the tank is vented to the vapor canister prior to refueling, the fuel vapor may saturate the canister to the point where fuel vapors from refueling cannot all be adsorbed within the canister. This may lead to an increase in bleed emissions, and may also lead to uncontrolled amounts of fuel vapor reaching the engine during a canister purge, potentially deviating the engine intake from commanded A/F ratio.
The inventors herein have recognized the above issues, and have developed systems and methods to at least partially address them. In one example, a system for an engine, comprising: a fuel tank; a thermal regulator comprising a phase-change material, the thermal regulator coupled to the fuel tank; and an engine coolant passage positioned to transfer thermal energy between engine coolant and the phase-change material. In this way, the temperature of the fuel tank may be managed passively by the phase-change material, and actively by engine coolant, thereby allowing heat energy to be shunted away from the fuel tank, cooling the fuel within the fuel tank, and reducing fuel vapor concentration, thus enabling the utilization of a fuel vapor canister with a reduced size.
In another example, a method for a vehicle, comprising: during a first condition, circulating engine coolant through a thermal regulator comprising a phase-change material, the thermal regulator coupled to a fuel tank. The first condition may comprise: an engine-on condition; and an ambient temperature above a first threshold. In this way, the phase-change material adsorbs heat from the fuel tank up to the phase-change temperature. Then, by circulating engine coolant through the thermal regulator, additional heat energy can be shunted away from the fuel tank. During operating conditions where a large amount of heat energy is rejected to the fuel tank, the fuel tank temperature may be stabilized, thus managing the fuel vapor concentration and fuel tank pressure.
In yet another example, a method for a vehicle, comprising: during a first condition, circulating engine coolant through a thermal regulator comprising a phase-change material, the thermal regulator coupled to a fuel tank; and then opening a fuel tank isolation valve. In this way, the fuel vapor concentration within a fuel tank may be reduced prior to a refueling event, decreasing the possibility of overwhelming the fuel vapor canister during depressurization, which may otherwise lead to an increase of bleed emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.